Digital x-ray imaging systems are becoming increasingly widespread for producing digital data which can be reconstructed into useful radiographic images. In current digital x-ray imaging systems, radiation from a source is directed toward a subject, typically a patient in a medical diagnostic application. A portion of the radiation passes through the patient and impacts a detector. The surface of the detector converts the radiation to light photons which are sensed. The detector is divided into a matrix of discrete picture elements or pixels, and encodes output signals based upon the quantity or intensity of the radiation impacting each pixel region. Because the radiation intensity is altered as the radiation passes through the patient, the images reconstructed based upon the output signals provide a projection of the patient's tissues similar to those available through conventional photographic film techniques.
Digital x-ray imaging systems are particularly useful due to their ability to collect digital data which can be reconstructed into the images required by radiologists and diagnosing physicians, and stored digitally or archived until needed. In conventional film-based radiography techniques, actual films were prepared, exposed, developed and stored for use by the radiologist. While the films provide an excellent diagnostic tool, particularly due to their ability to capture significant anatomical detail, they are inherently difficult to transmit between locations, such as from an imaging facility or department to various physician locations. The digital data produced by direct digital x-ray systems, on the other hand, can be processed and enhanced, stored, transmitted via networks, and used to reconstruct images which can be displayed on monitors and other soft copy displays at any desired location. Similar advantages are offered by digitizing systems which convert conventional radiographic images from film to digital data.
In certain type of imaging systems, such as digital x-ray systems, the radiation source may be positioned at various locations along an imaging area, with the detector typically being positioned at a corresponding location. For example, the source and detector may be moved along a longitudinal centerline of a patient support and, in certain systems, in a direction transverse to the centerline. Such positioning is useful for imaging specific anatomies or limbs, while exposing a patient to a minimal level of radiation.
In digital imaging systems, the computational load imposed on the image data processing circuitry is related to the amount of information collected. For larger or higher resolution images, or images employing a greater dynamic range for each pixel, significant quantities of data may be collected and processed to obtain the final data set used to reconstruct the image. Where smaller areas are imaged, such as specific anatomies in x-ray systems, electronic cropping may be used to reduce the total amount of data collected or processed. In general, such cropping entails selectively sampling or processing data from those pixels corresponding to the desired image area, the remaining pixels being considered to contain little or no usefull information.
For asymmetrical imaging (i.e. where the source is angularly positioned with respect to a projection line through the source and orthogonal to the detector and/or where the image center is not coincident with the detector center), no effective automated digital cropping technique has been developed. Consequently, in such cases, an operator or clinician may be required manually to view the image and crop the data after processing.
There is a need, therefore, for an improved image data cropping technique which will allow for automated digital cropping of image data in asymmetrical imaging situations. There is a particular need for an approach which permits the quantity of data sampled or processed to be reduced and which reduces the need for clinicians to manually view and crop resulting images.